LMH6723 Datasheet, PDF (18/28 Pages) National Semiconductor (TI) – Single/Dual/Quad 370 MHz 1 mA Current Feedback Op Amp

LMH6723, LMH6724
SNOSA83I – AUGUST 2003 – REVISED AUGUST 2014
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Inverting Input Parasitic Capacitance (continued)
Resistors R₄, R₅, R₆, and R₇ are necessary to ensure even current distribution between the amplifiers. Since they
are inside the feedback loop they have no effect on the gain of the circuit. The circuit shown in Figure 37 has a
gain of 5. The frequency response of this circuit is shown in Figure 38.
14
GAIN
13
12
11
PHASE
10
9
V_S = ±5V
8 _{VOUT = 2.5 VPP}
7 ^{RL = 5.6:}
A_V = 5
6
1
10
FREQUENCY (MHz)
180
135
90
45
0
-45
-90
-135
-180
100
Figure 38. Composite Amplifier Frequency Response
7.8 Layout Considerations
Whenever questions about layout arise, use the evaluation board as a guide. Evaluation boards are shipped with
sample requests.
To reduce parasitic capacitances ground and power planes should be removed near the input and output pins.
Components in the feedback loop should be placed as close to the device as possible. For long signal paths
controlled impedance lines should be used, along with impedance matching at both ends.
Bypass capacitors should be placed as close to the device as possible. Bypass capacitors from each rail to
ground are applied in pairs. The larger electrolytic bypass capacitors can be located anywhere on the board;
however, the smaller ceramic capacitors should be placed as close to the device as possible.
7.9 Video Performance
The LMH6723/LMH6724 has been designed to provide good performance with both PAL and NTSC composite
video signals. The LMH6723/LMH6724 is specified for PAL signals. Typically, NTSC performance is marginally
better due to the lower frequency content of the signal. Performance degrades as the loading is increased;
therefore, best performance will be obtained with back terminated loads. The back termination reduces
reflections from the transmission line and effectively masks transmission line and other parasitic capacitances
from the amplifier output stage. Figure 34 shows a typical configuration for driving a 75Ω cable. The amplifier is
configured for a gain of 2 to make up for the 6dB of loss in R_OUT.
7.10 Single 5-V Supply Video
With a 5V supply the LMH6723/LMH6724 is able to handle a composite NTSC video signal, provided that the
signal is AC coupled and level shifted so that the signal is centered around V_CC/2.
7.10.1 Application Curves
See Figure 31 through Figure 33 and Figure 38.
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